The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Circulating tumor cells (CTCs) are cells that sporadically release from a primary tumor and circulate in the bloodstream. Because CTCs travel in the blood stream, they may spawn growth of additional tumors in different tissues, creating a condition known as metastasis. CTCs, therefore, offer medical professionals a mechanism for identifying potential sources of tumor growth. Increasingly, CTCs are examined (in place of biopsies) during chemotherapy, radiation therapy, surgery, and other cancer treatments to assess treatment effectiveness and the likelihood of cancer spread.
There are three general techniques for CTC measurement. Size- and weight-based separation techniques generally require placing a blood sample in a test tube and isolating CTCs based on size, weight, or electric charge drift, also termed gradient centrifugation. Such techniques have various problems, stemming from the non-uniformity in size of CTCs and the inability to distinguish white blood cells from CTCs. Another technique uses microfilters, such as membrane filters, in an attempt to capture CTCs based on size differential. Yet another technique uses immunological methods to target and isolate CTCs. These latter techniques include magnetic separation techniques where magnetic beads are introduced into a cell mixture and bound with a CTC antigen, which is then examined. These latter techniques also include immunoaffinity techniques where a CTC is captured by a surface antigen bound to a substrate or post. In theory, the ability to count CTCs and test them using any of these techniques could help with diagnosis, prognosis, and determinations of preferred choices of treatment. In practice, however, the techniques are limited in effectiveness.
Many of the current CTC measurement techniques involve bulky and expensive setups. This is particularly true for immunological techniques, e.g., those based on expression of a putative tumor-associated biological marker or a tissue-type specific marker such as those of immuno-based or rtPCR-based strategies.
Moreover, all of the techniques are performed offline, in a laboratory, which slows measurement and often requires numerous CTC samples to achieve accurate measurement. Indeed, measurement times are a major limitation for CTC techniques. For one, CTCs move within the blood stream at such fast flow rates that capture is a difficult and inefficient process. CTC measurements, therefore, are performed externally to the body, for example, by using samples of blood collected over a period of time. But CTCs shed randomly and can die in the body, meaning that merely trying to draw blood and measure for CTC presence at any given point in time can result in false negatives, with existing systems. Additionally, the number of CTCs produced can depend on cancer type, which means that the number of samples needed to measure for CTC presence can vary, depending on the cancer type in the primary tumor.
Even with the most common CTC measurement technique, immunological techniques based on marker expression, current techniques are highly dependent upon the sensitivity and specificity of the reagents used for CTC marking (antibodies, rtPCR primers, invasive phenotype in collegen, etc.). The result is that techniques can miss upwards of 50%-100% of the CTCs in the blood, as that many CTCs may not express those markers, that includes cancer cells that might be the most biologically significant, such as cancer stem cells. The resulting biologic noise associated with low sensitivity and selectivity is thus a limiting factor, as is the low number of CTCs at any given time. Take, for example, 10 mL of blood, which may have billions of blood cells, billion of platets, millions of white blood cells, and anywhere from zero to a couple to a few hundred CTCs. To identify from among the main cancer types (breast, lung, colon, ovary, and prostate cancer, i.e., the “big five”), markers are chosen based on epithelial cell function. Epithelial cells produce keratin and Epithelial cell adhesion molecule (EpCAM), which can be expressed by markers. But not all cells that make cancer come from the epithelial cells form EpCAM. Some cancer cells will not make EpCAM, while other cancer cells may down-regulate EpCAM production until the cells have moved to a different location where they become active again. The result is that these biological-marker techniques may only see 50% of the CTCs.